Figure 1-4.Effects of ionospheric density on radio waves.Layer DensityFigure 1-4 shows the relationship betweenradio waves and ionization density. Each ionizedlayer has a middle region of relatively denseionization with less intensity above and below. Asa radio wave enters a region of increasingionization, a velocity increase causes it to bendback toward the earth. In the highly densemiddle region, refraction occurs more slowlybecause the ionization density is uniform. As thewave enters the upper less dense region, thevelocity of the upper part of the wave decreasesand the wave is bent away from the earth.FrequencyThe lower the frequency of a radio wave, themore rapidly the wave is refracted by a givendegree of ionization. Figure 1-5 shows threeseparate waves of differing frequencies enteringthe ionosphere at the same angle. You can see thatthe 5-MHz wave is refracted quite sharply, whilethe 20-MHz wave is refracted less sharply andreturns to earth at a greater distance than the 5-MHz wave. Notice that the 100-MHz wave is lostinto space. For any given ionized layer, there is afrequency, called the escape point, at which energytransmitted directly upward will escape intospace. The maximum frequency just below theescape point is called the critical frequency. Inthis example, the 100-MHz waves frequency isgreater than the critical frequency for that ionizedlayer.Figure 1-5.Frequency versus refractionand distance.The critical frequency of a layer depends uponthe layers density. If a wave passes through a1-5